The cast of Sherlock are returning in first episode The Six Thatchers (Picture: Todd Antony/BBC)

As Sherlock series four gears up to be the first blockbuster drama of 2017, we’ve gathered everything you can expect from the first episode.

It’s been almost a year since we last journeyed into the mind palace of Benedict Cumberbatch’s Sherlock, with special one-off The Abominable Bride serving as a great historical deviation to bridge into the next full series.

While the wait has been unbearably lengthy, we’re finally on the home stretch back to Baker Street – so here’s everything you can expect from highly-anticipated series four opener, The Six Thatchers. Metro.co.uk was treated to a sneak preview, and here’s what we learned:

1. The show’s about to get much darker

As the face of Moriarty looms large in Sherlock’s mind throughout The Abominable Bride – it doesn’t spell the most happiest of times for the detective and his dear John Watson.

Considering series three was relatively lighter and funnier than previous outings, it’s pleasing to see we’re treading back into ‘tougher’ territory.

If you’re looking for clues as to Sherlock’s role in the new dynamic however, the last sentence of the post remarking how he’s ‘always on his phone’ might give a sneaky indication as to his manic work ethic in the opening episode.

3. Sherlock’s getting a bloodhound at his side

Sherlock gets a new furry friend (Picture: BBC)

As previously revealed in an early on-set picture, Sherlock is seen with a trusty hound at his side which helps the detective in the opening episode.

While the mutt certainly looks primed for action – let’s just say he isn’t quite so helpful, even if he does appear in one of the hilarious stand-out scenes of the opener.

4. The episode’s loosely based on The Adventure Of The Six Napoleons

Benedict Cumberbatch is getting hairy in this series of Sherlock (Picture: YouTube)

The Six Thatchers is loosely based around events in Sir Arthur Conan Doyle’s story, The Adventures Of The Six Napoleons – which features plaster busts of the French military leader being destroyed in search of a hidden pearl.

We don’t want to leave too many breadcrumbs, but you’ve probably already worked out there’s a certain former UK prime minster who matches the title. We’ll just emphasise the term ‘loosely’, as there’s a lot more going on inside those busts than meets the eye.

5. You won’t be disappointed

Managing to seamlessly juggle the kinetic humour the show’s become synonymous with alongside its darker plunges, The Six Thatchers is a memorable Sherlock tale which also paves an intriguing, twisted stage for episodes to come.

In other words, it’s very, very good. You should tune in.

6. Come on then – when’s it on?

You can catch Sherlock’s return New Year’s Day on BBC One at 8.30pm. Prime the sofa.